The present invention relates to an automatic air-fuel ratio control apparatus for use in a combustion system of a type in which combustion air is preheated, in a heat exchanger, by exhaust gas at a high temperature in order to improve the thermal efficiency of the system.
Commonly, in a combustion system of a type in which combustion air and fuel are respectively supplied under a constant pressure, the flow control for the preheated combustion air and the fuel is accomplished by adjusting corresponding values so as to obtain the best air-fuel ratio for thermal efficiency. The best air-fuel ratio for thermal efficiency is approximately in the range of 1.1 to 1.2, depending on a type of burner and a type of fuel.
Meanwhile, the temperature of the preheated combustion air is not constant during operation, because the flow rate of the combustion air is variably controlled on the basis of the desired operation of the combustion system and consequently the temperature of the exhaust gas varies, so that the density of the combustion air changes. Accordingly, even if the opening of the air flow control valve of the burner remains constant so as to keep a constant volume of the combustion air flow, the mass of the combustion air flow varies in accordance with the temperature variation of the combustion air. The mass of the combustion air flow Q is expressed by the following formula: EQU Q=.sqroot..alpha..sub.o .times.V EQU .alpha..sub.o =Ko/K
wherein
V : volume of combustion air flow, PA1 Ko: standard absolute temperature, PA1 K : actual absolute temperature.
Accordingly, under the condition that the volume of the combustion air flow is constantly controlled, the air-fuel ratio varies in accordance with the temperature of the combustion air. For instance, when the temperature of the combustion air changes from 20.degree. C. to 250.degree. C., the air-fuel ratio consequently changes from 1.1 to 0.75. As a result, in spite of the fact that the combustion air is preheated for the purpose of improvement of the thermal efficiency, it causes the deterioration of the thermal efficiency or inoperation of the combustion system and further creates a pollution nuisance.
In order to avoid such problems, commonly there has been provided an automatic air-fuel ratio control apparatus which constantly controls the air-fuel ratio after measuring the flow of the preheated combustion air flow in consideration of the temperature thereof.
The conventional air-fuel ratio control apparatus, however, has such a drawback that it needs expensive metering instruments such as a flowmeter for measuring the flow of the combustion air, a calculator for calculating the corresponding proper flow of combustion air in accordance with the temperature of the combustion air at every time, etc.
Conventionally, there has been provided an inexpensive air-fuel ratio control apparatus, for example, in Japanese Laid Open Patent Application Kokai No. 53-41831. This type of air-fuel ratio control apparatus, however, has an oversized and complicated configuration and, also requires a complicated operation for changing the air-fuel ratio, since it is directly incorporated in a fuel supply conduit.